Impact of Arsenic-Rich Soil and Groundwater Production of Selected Crops and Remediation Strategies for Reducing Arsenic Content in Crops

Abstract

Arsenic (As) a toxic substance in the environment is a major public health concern worldwide High concentrations of As have also been linked to cancers of the nasal cavity lung liver bladder kidney and prostate and can lead to hypertension and diabetes Peripheral vascular gangrene also known as Blackfoot disease (BFD) was first reported in Chianan Plain of southwestern Taiwan in the 1960s Since 1980 As-rich groundwater has no longer been consumed as drinking water in Taiwan although it is still widely used for irrigation aquacultural and industrial purposes and hence deserves our continued attention Rice is the staple food for nearly one-half of world’s population including those living in Taiwan China Japan Korea and other Asian countries However rice uptakes As into grains are much more readily occurred than other cereal crops The present study geochemically investigates As-rich groundwater soil and rice plants from paddy fields in Chianan Plain of southwestern Taiwan an area which is mainly used for rice cultivation The stable isotopes of oxygen-18 and deuterium were used to identify different sources of water in a soil layer in rice paddy during the rice growing season in 2014 This study can help us understand that the level of As in rice plants can be affected by the groundwater used for irrigation type and concentration of As in the soil and soil properties of paddy fields In addition experimental results by means of stable isotopes technique clarified the infiltration of rainfall in the complicated process that can be affected by soil structure texture moist and extent of heterogeneity Results show that the total As concentrations in the groundwater used for irrigation of the sampled paddy fields at Hsuechia Yichu and Budai in the Chianan Plain are in the range of 32 9 to 175 7 μg L–1 which is higher than the permissible drink limit (10 μg L–1) recommended by the World Health Organization (WHO) and irrigation limit (50μg L–1) recommended by agricultural authorities of Taiwan The percentages of As in different parts of the rice plants found in the current study are in the range of 88 3 to 92 8% in roots 2 8 to 4% in shoots 1 5 to 5 2% in leaves 1 to 1 7% in husks and 0 8 to 1 2% in grains This study investigated the impacts of various types of irrigation practices with As-contaminated groundwater on the extent of As accumulation within rice plant parts during development and rice crop production at maturity Three types of irrigation practices were applied to As-rich paddy fields: flooded irrigation aerobic irrigation and alternate wetting and drying irrigation (AWDI) Results show that the arsenic concentration in different parts of rice plants varied with growth stage and irrigation practices in both cropping seasons Lower levels of As in rice were found in AWDI and aerobic irrigation than in flooded irrigation Different irrigation practices can change the oxidation and reduction conditions of the paddy field which lead to As release or absorption in the soil thus influencing the uptake of As by plants The chemical immobilization of As-rich soil by using ethylenediaminetetraacetic acid ferric sodium salt (EDTA-Fe) and calcium peroxide (CaO2) as stabilizing agents was investigated in Chianan Plain of southwestern Taiwan The As-rich soil was amended with EDTA-Fe at the rates of 0 0 35 0 7 and 1 4 Mg/ha or with CaO2 at the rates of 0 0 38 0 76 and 1 52 Mg/ha and grown with radish (Raphanus sativus L ) lettuce (Lactuca sativa) Chinese cabbage (Brassica rapa) and Arden lettuce (Lactuca sativa L ) Results showed that those amended with EDTA-Fe at 0 35 and 0 7 Mg ha?1 can significantly reduce As accumulation in vegetables Moreover the uptake of As in vegetables decreased more in soil amended with CaO2 relative to that amended with EDTA-Fe As indicated applications of EDTA-Fe at the rate of 0 7 Mg ha?1 and CaO2 at the rate of 1 52 Mg ha?1 can obtain optimal effect on suppressing As uptake by vegetables Present study aimed to assess the presence and availability of As in paddy field of the Chianan Plain Arsenic content was determined in soils and pore water sampled at 5 sampling depths (20 40 60 80 90 cm) in paddy field The As concentrations in the experimental field soil varied slightly with sampling depths Total As concentration positively and significantly (p <0 05) correlated with the soil properties including free Fe (Fed) free Mn (Mnd) amorhous Fe (Feo) amorhous Mn (Mno) cation capacity exchange and clay content The sequential extraction of soil As showed that As was mainly fixed by both amorphous and crystalline Fe/Mn oxides which may be ascribable to that As has a high affinity for Fe/Mn oxides under reduction regime of groundwater As a consequence As probably would become soluble due to the usage of groundwater and the change of redox regime In addition coarse texture of study soil the flow path of irrigation water and the gravity impact are factors affecting As movement in the soil layers The soil As was concentrated on the surface soil and gradually decreased with soil depth Therefore total As in subsoils was lower than topsoil because As was adsorbed and accumulated by abundant Fe/Mn oxides in surface soil after irrigation Based on the measured deuterium and oxygen-18 in soil water rainwater and groundwater in the paddy field of the Chianan Plain of southwestern Taiwan in the wet season the relationship between δD and δ18O in soil water and groundwater recharge after typhoon rainfall (event water) was investigated in the present study The soil water at different depths before and after event water varied in hydrogen and oxygen isotope ratios The top soil layer (<30 cm depth) had more depleted isotopic compositions as a result of the higher rate of evaporation Similar soil water isotope composition profiles were observed in shallow soil layers More depleted fractions of isotopes were found in groundwater as those in rainwater suggesting that the groundwater primarily came from the rainwater However the isotope compositions of hydrogen and oxygen in groundwater is still slightly deviated from the local meteoric water line in southwestern Taiwan The proposed conceptual model for the interaction among soil plant and arsenic in the study area can help understand the mechanism that arsenic was released in the subsurface environment The remedial measure and strategy for reducing arsenic content in selected crops is also proposed in this research

Date of Award	2016 Aug 3
Original language	English
Supervisor	Jiin-Shuh Jean (Supervisor)